In the patent EP 1 754 766 B1, an aqueous resin for dispersion adhesives is disclosed which comprises two polymers, whereof the first polymer having a glass transition temperature of from 0° C. to 60° C. is obtained by emulsion polymerisation of a monomer composition comprising a mass fraction of from 80% to 99.99% of at least one monomer selected from the group consisting of aromatic vinyl monomers and (meth)acrylate monomers which are esters of hydroxy-functional compounds and acrylic or methacrylic acid, a mass fraction of up to 1% of crosslinkable monomers, and a mass fraction of up to 19.99% of different monomers which may also comprise hydroxy-functional vinyl monomers, and the second polymer having a glass transition temperature of from −40° C. to 20° C. is obtained by emulsion polymerisation of a monomer composition comprising a mass fraction of from 80% to 99.4% of at least one monomer selected from the group consisting of aromatic vinyl monomers and (meth)acrylate monomers which are esters of hydroxy-functional compounds and acrylic or methacrylic acid, a mass fraction of from 0.5% to 5% of hydroxy-functional vinyl monomers, a mass fraction of from 0.1% to 5% of olefinically unsaturated carboxylic acid monomer, and a mass fraction of up to 19% of any other monomer.
In the patent application JP Sho 49-109 426 A, water-based thermosetting coating compositions are described which comprise a copolymer made from a polyalkylene glycol acrylate, an olefinically unsaturated carboxylic acid, and further vinyl monomers, and an aminoplast crosslinker.
In the Japanese patent application 2010-284 465 A, a powdery acrylic pressure-sensitive adhesive composition is disclosed which contains acrylic emulsion copolymer particles, and a crosslinking agent in an aqueous medium. The mass fraction of hydroxy-functional monomers in the monomer mixture is from 0.01% to 5%.
In the Japanese patent application 2005-179 614 A, an aqueous copolymer emulsion is described made from an unsaturated hydroxy-functional vinyl monomer, and other copolymerisable (meth)acrylic monomers. The mass fraction of hydroxy-functional monomers in the monomer mixture is between 5.6% and 5.7% (working example 1, preparations (a) and (a′)), and the mass fraction of alkyl(meth)acrylates in these preparations is between 89.5% and 89.6%.
Two-pack coating compositions based on solvent-borne acrylic resins have been used widely in the coatings industry, particularly in coating of substrates that are not to be exposed to high temperature for curing, such as automotive refinish paints. Multifunctional isocyanates are mostly used as curing agents for these hydroxy-functional resins. As there is a trend to replace solvent-borne coating compositions by water-borne, or aqueous, coating compositions to reduce the emission of solvents during film formation and drying, a transition is seen towards aqueous systems. Isocyanates are well-known to be prone to react with water under formation of carbon dioxide, therefore, small bubbles can be formed in such aqueous coating compositions which are particularly visible in coating layers of higher thickness, such as 50 μm or more.
It is therefore an object of the invention to provide aqueous coating compositions based on hydroxy-functional acrylic copolymer dispersions and curing agents therefor, with a reduced propensity to formation of bubbles.
In thicker layers, sagging is also frequently observed. Such sagging can be controlled by addition of thickeners or rheology additives. Rheology additives based on finely dispersed silica, for example, reduce the propensity to sagging, but show varying degree of gloss reduction in dependence of the coating layer thickness. Low gloss, or matt coatings have recently met with increased demand. While addition of matting agents such as finely dispersed silica leads to coating films with reduced gloss, it has, however, been observed that the reduction of gloss is dependent on the coating layer thickness, apparently due to the higher number of scattering particles in a thicker layer. A coating composition is therefore needed that does not exhibit these shortcomings of the presently available systems.
In the experiments underlying the present invention, it has been found that a coating composition comprising a dispersion of a hydroxy-functional acrylic copolymer having emulsifiers chemically attached to the polymer chain, and a curing agent which is preferably based on multifunctional isocyanates, provides a coating film which is essentially free from bubbles up to coating layer thickness of 200 μm, has a low propensity to sagging, and has a reduced gloss which is independent on the thickness of the coating film. The emulsifiers which can be used in the present invention have at least one carbon-carbon double bond which is copolymerisable with the olefinically unsaturated monomers on which the hydroxy-functional acrylic copolymer is based.